linux-kernel - Re: [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting max

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Message-ID: <ZYGUOslxxwe1sNzR@casper.infradead.org>
Date: Tue, 19 Dec 2023 13:01:46 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Jingbo Xu <jefflexu@...ux.alibaba.com>
Cc: shr@...kernel.io, akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, joseph.qi@...ux.alibaba.com,
	linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting
 max_ratio

On Tue, Dec 19, 2023 at 01:58:21PM +0800, Jingbo Xu wrote:
> On 12/19/23 12:06 PM, Matthew Wilcox wrote:
> > On Tue, Dec 19, 2023 at 10:42:46AM +0800, Jingbo Xu wrote:
> >>  	} else {
> >>  		bdi->max_ratio = max_ratio;
> >> -		bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
> >> +		bdi->max_prop_frac = div64_u64(FPROP_FRAC_BASE * max_ratio,
> >> +					       100 * BDI_RATIO_SCALE);
> >>  	}
> > 
> > Why use div64_u64 here?
> > 
> > FPROP_FRAC_BASE is an unsigned long.  max_ratio is an unsigned int, so
> > the numerator is an unsigned long.  BDI_RATIO_SCALE is 10,000, so the
> > numerator is an unsigned int.  There's no 64-bit arithmetic needed here.
> 
> Yes, div64_u64() is actually not needed here. So it seems
> 
> bdi->max_prop_frac = FPROP_FRAC_BASE * max_ratio / 100 / BDI_RATIO_SCALE;
> 
> is adequate?

I'd rather spell that as:

		bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) /
					(100 * BDI_RATIO_SCALE);

It's closer to how you'd write it out mathematically and so it reads
more easily.  At least for me.